Established symmetrical encryption methods require the presence of a shared secret at the participating communication partners. Either this shared secret is used directly as the key, or the key is derived therefrom. The encrypted communication is considered secure if the key is random and sufficiently long. In AES, for example, 128 bits is considered long enough to make cracking the process appear futile.
To provide a shared secret key to the communication partners, conventionally an asymmetrical cryptosystem such as the Diffie-Hellman, RSA or Elgamal key exchange method is used. However, these methods are susceptible to man-in-the-middle attacks or require the presence of an infrastructure having trust points. In addition, these methods are highly complex, and also require real random number generators at the communication partners. Small devices such as embedded sensors which are intended to communicate in an encrypted manner often have neither sufficient computing power nor actual physical sources of randomness.
An elegant method from the field of physical layer security for generating shared secret keys for wireless communication is key generation using the reciprocity of multi-path channels. In this context, the secret key is generated directly from the properties of the wireless communication channel. Kui Ren, Hai Su, Qian Wang: “Secret key generation exploiting channel characteristics in wireless communications”, Wireless Communications, IEEE, vol. 18, No. 4, pp. 6, 12, Aug. 2011 and Azimi-Sadjadi, Babak, Aggelos Kiayias, Alejandra Mercado and Bulent Yener, “Robust key generation from signal envelopes in wireless networks”, in Proceedings of the 14th ACM conference on computer and communications security, pp. 401-410, ACM, 2007, describe known methods by which secret keys, for example for symmetrical encryption methods, can be extracted from the random properties of a radio channel.
A prerequisite for this is that the wireless channel is reciprocal and has sufficient random properties. Furthermore, the channel must be highly location-dependent, within the meaning that any listeners in the vicinity of the communication partners establish or obtain different physical channel properties. Random, in other words unpredictable, physical channel properties result for example from signal superposition due to multi-path propagation. At the relevant receiver, this leads to a unique signal pattern which can be used to generate random bits for a secret key.
There is therefore a need to specify an improved method for generating a digital key in two communication devices by exploiting the reciprocity of the radio channel.